Many embedded applications read analog inputs and so have to convert those inputs to digital results. An ADC accepts an analog input signal (typically a voltage or a current), samples it, and converts each sample to a digital value that can be read, e.g., by a microcontroller, a microprocessor, a digital circuit, etc. Generally, the measurable output of an ADC is the (Vin/Vref)×Resolution. Resolution is a characteristic of an ADC, and may be expressed as the quantum of the input analogue voltage change required to increment an ADC's digital output from one value to the next higher code value. For example, the resolution of an 8-bit ADC may be expressed as one part in 255 or as 0.4% of full scale or simply as 8-bit resolution. So, if such an ADC has a full-scale analogue input signal range of 10 V then it may resolve a 40 mV change in input signal. The step size is the voltage difference between one digital level (i.e., 0001) and the next one (i.e., 0010 or 0000). For example, if an ADC has a step size of 40 mV, an input of 40 mV will produce an output in an 8-bit converter of 0000 0001.
Thus, to measure and convert small signal changes accurately, conventional ADCs use a higher resolution (i.e., capability to resolve a small mV change in input) while still being able to measure and convert large signal changes accurately (i.e., capability to resolve a larger mV change in input). For a conventional ADC that is expected to measure a wide dynamic range the measurements are relatively coarse when measuring small analog signals, that is the relative step size between each digital code is large when measuring small analog signals, while relatively fine when measuring large analog signals, that is the relative step size between each digital code is relatively small.
Many embedded applications that read an analog input signal require a large dynamic range and accurate measurements, that is, the ability to measure from a low voltage (e.g., the ground voltage) up to a high voltage (e.g., up to the system supply voltage)—while doing so accurately. For this reason, many applications need increasing ADC resolution. Conventional embedded applications typically, therefore, use a high-resolution ADC—e.g., 12-bit, 14-bit or higher—or an internal, programmable, gain stage for the ADC, so that small signals can be amplified as shown in FIG. 1.